Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell (see Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995).
In general, protein kinases mediate intracellular signaling by affecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. These phosphorylation events are ultimately triggered in response to a variety of extracellular and other stimuli. Examples of such stimuli include environmental and chemical stress signals (e.g. shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g. interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-a), and growth factors (e.g. granulocyte macrophage-colony stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, survival and regulation of the cell cycle.
Kinases may be categorized into families by the substrates they phosphorylate (e.g. protein-tyrosine, protein-serine/threonine, lipids etc). Sequence motifs have been identified that generally correspond to each of these kinase families (See, for example, Hanks, S. K., Hunter, T., FASEB J. 1995, 9, 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al, Cell 1992, 70, 419-429; Kunz et al, Cell 1993, 73, 585-596; Garcia-Bustos et al, EMBO J 1994, 13, 2352-2361).
A serine/threonine kinase, protein kinase C-theta (PKC-theta), is a member of the novel, calcium independent PKC subfamily that is selectively expressed in T cells and skeletal muscle. Several lines of evidence indicate that PKC-theta has an essential role in T cell activation. Upon antigen stimulation of T cells, PKC-theta, but not other PKC isoforms, rapidly translocates from the cytoplasm to the site of cell contact between the T cell and antigen-presenting cell (APC), where it localizes with the T cell receptor (TCR) in a region termed the central supramolecular activation cluster (cSMAC) (Monks et al., 1997, Nature, 385: 83-86; Monks et al., 1998, Nature, 395: 82-86).
It has been reported that PKC-theta selectively activates the transcription factors AP-1 and NF-κB and integrates TCR and CD28 co-stimulatory signals leading to the activation of the CD28 response element (CD28RE) in the IL-2 promoter (Baier-Bitterlich et al., 1996, Mol. Cell. Biol., 16: 1842-1850; Coudronniere et al., 2000, PNAS, 97: 3394-3399). The specific role for PKC-theta in CD3/CD28 co-stimulation of T cells is highlighted in a study where expression of a kinase-dead PKC-theta mutant, or anti-sense PKC-theta dose-dependently inhibited CD3/CD28 co-stimulated NF-κB activation, but not TNF-alpha-stimulated NF-κB activation. This was not seen with other PKC isoforms (Lin et al., 2000, Mol. Cell. Biol., 20: 2933-2940). Recruitment of PKC-theta to the SMAC is reported to be mediated by its N-terminal regulatory domain and is necessary for T cell activation, as an over-expressed PKC-theta catalytic fragment did not translocate and was unable to activate NF-κB, whereas a PKC-theta catalytic domain-Lck membrane-binding domain chimera was able to reconstitute signaling (Bi et al., 2001, Nat. Immunol., 2:556-563).
Translocation of PKC-theta to the SMAC appears to be mediated by a largely PLC-gamma/DAG-independent mechanism, involving Vav and PI3-kinase (Villalba et al., 2002, JCB 157: 253-263), whilst activation of PKC-theta requires input from several signaling components including Lck, ZAP-70, SLP-76, PLC-gamma, Vav and PI3-kinase (Liu et al., 2000, JBC, 275: 3606-3609; Herndon et al., 2001, J. Immunol., 166: 5654-5664; Dienz et al., 2002, J. Immunol., 169: 365-372; Bauer et al., 2001 JBC., 276: 31627-31634). These biochemical studies in human T cells have gained credence from studies in PKC-theta knockout mice, which have confirmed a crucial role for this enzyme in T cell function. PKC-theta−/− mice are healthy and fertile, have a normally developed immune system, but exhibit profound defects in mature T cell activation (Sun et al., 200, Nature, 404:402-407). Proliferative responses to TCR and TCR/CD28 co-stimulation were inhibited (>90%) as were in vivo responses to antigen. In agreement with studies on human T cells, activation of the transcription factors AP-1 and NF-κB was abrogated, resulting in a severe deficit in IL-2 production and IL-2 R upregulation (Baier-Bitterlich et al., 1996, MBC, 16, 1842; Lin et al., 2000, MCB, 20, 2933; Courdonniere, 2000, 97, 3394). More recently, studies in PKC-theta-deficient mice have indicated a role for PKC-theta in the development of mouse models of autoimmune diseases, including multiple sclerosis (MS), rheumatoid arthritis (RA) and irritable bowel disease (IBD) (Salek-Ardakani et al., 2006; Tan et al., 2006; Healy et al., 2006; Anderson et al., 2006). In these models, PKC-theta-deficient mice exhibited a marked reduction in disease severity that was associated with a profound defect in the development and effector function of autoreactive T cells.
In addition to its role in T cell activation, PKC-theta is reported to mediate the phorbol ester-triggered survival signal that protects T cells from Fas- and UV-induced apoptosis (Villalba et al., 2001, J. Immunol. 166: 5955-5963; Berttolotto et al., 2000, 275: 37246-37250). This pro-survival role is of interest because the human PKC-theta gene has been mapped to chromosome 10 (10p15), a region associated with mutations leading to T cell leukaemias and lymphomas (Erdel et al., 1995, Genomics 25: 295-297; Verma et al., 1987, J. Cancer Res. Clin. Oncol., 113: 192-196).
In vivo, the role for PKC-theta in immune responses to infection is dependent on the type of pathogen encountered. PKC-theta deficient mice elicit normal Th1 and cytotoxic T cell-mediated responses to several viral infections and the protozoan parasite, Leishmania major and effectively clear these infections (Marsland et al., 2004; Berg-Brown et al., 2004; Marsland et al., 2005; Giannoni et al., 2005). However, PKC-theta deficient mice are unable to wage normal Th2 T cell responses against the parasite Nippostrongylus brasiliensis and certain allergens (Marsland et al., 2004; Salek-Ardakani et al., 2004) and are unable to clear Listeria monocytogenes infection (Sakowicz-Burkiewicz et al., 2008). Clearly in some circumstances, the requirement for PKC-theta in T cell activation can be bypassed and this is likely to involve the provision of additional signals to T cells, either from cells of the innate immune system, or directly from the pathogen in the form of pathogen associated molecular patterns (PAMPs) (Marsland et al., 2007).
More recently, studies in PKC-theta-deficient mice have indicated a role for PKC-theta in the development of mouse models of autoimmune diseases, including multiple sclerosis, rheumatoid arthritis and inflammatory bowel disease. In all cases where examined, PKC-theta-deficient mice exhibited a marked reduction in disease severity that was associated with a profound defect in the development of a newly discovered class of T cells, Th17 cells (Salek-Ardakani et al., 2006; Tan et al., 2006; Healy et al., 2006; Anderson et al., 2006; Nagahama et al., 2008). PKC-theta therefore appears to be essential for the development of pathogenic auto-reactive Th17 cells in the context of autoimmunity. These observations support the notion that targeting PKC-theta will provide a way to target autoimmune T cell responses, leaving many T cell responses (e.g., to viral infections) intact.
In addition to its role in T cell activation, PKC-theta mediates the phorbol ester-triggered survival signal that protects T cells from Fas- and UV-induced apoptosis (Villalba et al., 2001, J. Immunol. 166: 5955-5963; Berttolotto et al., 2000, 275: 37246-37250). This pro-survival role is of interest because the human PKC-theta gene has been mapped to chromosome 10 (10p15), a region associated with mutations leading to T cell leukaemias and lymphomas (Erdel et al., 1995, Genomics 25: 295-297; Verma et al., 1987, J. Cancer Res. Clin. Oncol., 113: 192-196).
Together, these data indicate that PKC-theta is an attractive target for therapeutic intervention in inflammatory disorders, immune disorders, lymphomas and T cell leukaemias.
Accordingly, there is a great need to develop compounds useful as inhibitors of protein kinases. In particular, it would be desirable to develop compounds that are useful as inhibitors of PKC-theta, particularly given the inadequate treatments currently available for the majority of the disorders implicated in their activation.